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It was shown by Hamilton in the 1990s that the isoperimetric ratio $C_H$ on the $2$-sphere improves along the Ricci flow.

A way to prove this is to use the fact that if $(M^2, g(t))$ is a solution of the Ricci flow on a topological $2$-sphere, then the isoperimetric ratios $C_H (\rho , t)$ of parallel loops $\gamma_{\rho}$ measured with respect to the metric $g(t)$ satisfy a heat-type equation

$\frac{\partial }{\partial t} (\log C_H) = \frac{\partial^2}{\partial {\rho}^2 } (\log C_H) + \frac{\Gamma}{L} \frac{\partial}{\partial \rho} (\log C_H)+ \frac{4 \pi - C_H}{A} \bigg( \frac{A+}{A_-} + \frac{A_-}{A_+} \bigg) $,

where $\Gamma$ is an integral of the signed curvature and more definitions can be found in the textbook of Chow and Knopf.

The isoperimetric ratio $C_H$ is similar to, but distinct from the Cheeger isoperimetric constant.

1.) Is it known if the Cheeger constant $h$ also satisfies a heat-type equation for a solution of the Ricci flow on a topological $2$-sphere?

2.) For a topological $2$-sphere, is it the case that

$C_H (M) \leq h (M)?$

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1 Answer 1

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Answer to (2) is negative. For a counterexample, consider a $2$-sphere and blow up the radius. The isoperimetric ratio $C_H$ remains the same, but the Cheeger constant $h$ shrinks, essentially because $C_H$ is a dimensionless quantity which is independent of the scaling, whereas $h$ is not.

Actually in Lemma 5.85 of the textbook of Chow and Knopf, it is shown that if $(M^2, g)$ is a closed orientable Riemannian surface, then

$C_H (M) \leq 4 \pi,$

which is obviously not true for the Cheeger constant.

I did a few calculations and looks as if answer to (1) is positive.

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    $\begingroup$ About one year ago, I feel the $h(M)$ maybe monotone along Ricci flow before singularity. But I don't know how to prove it, even though on special manifold. Since I just be a beginner of Ricci flow, I give up it. But I still be interested in it. If possible, could you tell me the calculation about (1). Thanks very much. $\endgroup$
    – Enhao Lan
    Commented Feb 15, 2023 at 12:06
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    $\begingroup$ @EnhaoLan Hi Enhao, did you maybe want to send me an email as might be easier to explain there. $\endgroup$ Commented Oct 20 at 14:48
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    $\begingroup$ Thank you. I read your paper last year. There are a few details I don't understand. I'll send you an email in three weeks, because I have some unfinished business, and I also need some time to recall your paper. Thanks again. $\endgroup$
    – Enhao Lan
    Commented Oct 21 at 11:48
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    $\begingroup$ @EnhaoLan Great, I look forward to this discussion. $\endgroup$ Commented Oct 21 at 23:43
  • $\begingroup$ Your mailbox rejected my email. But my question is very simple. In your new paper (arxiv.org/pdf/2404.13063), I don't see the equivalence between (3) and (4). Although they differ only twice on the round sphere, there seems to be no definite proportional relationship between them on the ordinary topological sphere. $\endgroup$
    – Enhao Lan
    Commented Nov 8 at 9:31

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